In the conventional "activated sludge" process for the treatment of sewage, an aeration tank of substantial size is followed by a large settling tank. The material being treated is aerated in the first tank for a period of time sufficient to achieve, through the action of oxygen and biological life such as aerobic bacteria and protozoa, desired biochemical removal of organic waste. In the settling tank, as its name indicates, sludge settles to the lower portion of the tank and a relatively clear effluent rises to the top.
Sewage or other aqueous waste material containing solid particles suspended therein is introduced at the inlet of the aeration tank where, together with activated sludge that has been returned to the tank in the start-up phase of the continuous process, it forms "mixed liquor." Mixed liquor contains not only aqueous waste material but also an active "biomass" that has been in contact with the aqueous waste material in the presence of oxygen for respiration. After it has remained in the aeration tank for a period of time, mixed liquor is continuously withdrawn from the outlet of the tank and delivered to the settling tank.
In the settling tank, a substantial portion of the suspended solids settles out as sludge (including as its volatile portion the biomass referred to above), and is withdrawn, to be either returned to the aeration tank as "activated sludge" or wasted as excess. The clarified supernatant liquid is drawn off from the top of the settling tank and discharged as secondary effluent.
The ultimate products of the activated sludge process of sewage treatment are carbon dioxide, water, synthesized biomass or synthesized cell material, and minor amounts of nitrogen. The purpose of the settling tank is to provide settled activated sludge containing aerobic biological life (such as bacteria and protozoa) for return to the aeration tank in order to give the biological life repeated opportunities to metabolize the organic waste. Other material on which the aerobic bacteria and other biological life can feed, including synthesized cell material and soluble organic matter, is also returned from the settling tank to the aeration tank as part of the activated sludge.
In order to promote the biochemical removal of organic waste by aerobic bacteria and other biological life as just described, it is necessary to provide sufficient oxygen to support the aerobic biological activity. To this end, in the aeration stage of the typical activated sludge process of sewage treatment, bubbles of air, usually of a diameter of about 1 to 2 mm. or larger, are introduced into the mixed liquor in the aeration tank. Conventionally, this is accomplished through the use of mechanical aerators to spray the liquid into contact with the air above the surface of the body of liquid, or through the use of spargers or porous media air diffusers below the surface to bubble air up through the body of liquid.
The introduction of such bubbles sometimes produces a foam or froth of intermixed air bubbles and largely untreated solid particles, which rises to the top of the aeration tank, usually at or near the front end thereof. This result has been considered to be very undesirable, and every effort has been made to avoid the formation of foam or froth containing untreated solids as described, because the two purposes of the aeration tank have been (1) to achieve thorough and extended mixing between the suspended solids and the gas bubbles or the dissolved gas, and (2) to deliver a suspension of treated solids to the settling tank for removal of those solids there through the settling process. Thus, water sprays have often been used to combat foaming in the aeration tank by collapsing any foam or froth formed on the surface of the material in the tank, in order to cause the primarily untreated sludge to resubmerge in the aqueous medium in the aeration tank for further treatment there.
Another undesirable form in which solid material may rise to the surface of a sewage treatment tank is the floated sludge, or "float," that is sometimes produced in an improperly operated settling tank. If the sludge that has settled in the settling tank is left too long on the bottom of the tank, it becomes anaerobic through the formation of carbon dioxide, methane, and other gases. Bubbles of these gases attach themselves to some of the solid particles of the sludge, and cause the particles to rise to the top of the settling tank. When this happens, the resulting float must be skimmed off the contents of the settling tank in order that it will not pass out from the tank as a part of the plant effluent.
Special flotation tanks have sometimes been employed in sewage treatment installations, but only after -- not before -- the aqueous material under treatment has passed through a settling tank. Such special flotation tanks are located in the waste sludge line through which a portion of the settled activated sludge that has been removed from the settling tank is discharged instead of being returned for introduction into the aeration tank. The float removed from the top of such a special flotation tank in the waste sludge line has a higher percentage of solid matter (usually about 4 percent by weight), than the settled sludge withdrawn from the settling tank (which may be up to about 2 percent by weight), and thus is more suitable for disposal as waste. This more concentrated sludge is sometimes further concentrated, as for example by use of a vacuum filter, to something like 15 to 20 percent by weight for its ultimate disposal.
These percentages are achieved in most instances through the addition of suitable polymers to encourage the agglomeration of the suspended solids to form larger and more easily floated floc particles. In the vacuum filtration process of final concentration of waste sludge, dewatering agents (such as ferric chloride and lime) are also commonly added to assist in the formation of a denser filter cake.
In special flotation tanks that follow the settling tank as just described, it has been common to introduce air bubbles either by a "dissolved air flotation" technique or by a "dispersed air flotation" technique. U.S. Pat. Nos. 3,063,938 and 3,525,437 disclose examples of the first of these techniques used in a sewage treatment process. U.S. Pat. Nos. 1,985,153, 2,055,065 and 2,246,559 disclose examples of the second technique, used in the patents in question in an ore flotation process.
In the dissolved air flotation technique, pressurized liquid with air dissolved therein (usually under a pressure of about 40 to 60 p.s.i.g.) is introduced into the activated sludge as it is pumped from the settling tank to the special flotation tank. When the liquid medium thus formed is released into the contents of the flotation tank at some location below the surface of those contents, where it is ordinarily subjected to only somewhat more than zero gauge pressure (i.e., somewhat more than one atmosphere absolute pressure), the dissolved air comes out of solution in the form of tiny bubbles, usually about 50 to 100 microns in diameter, which attach themselves to some of the solid particles and cause a float to rise to the top of the tank. In this system, the pressurized liquid in which air is dissolved is typically the effluent from the special flotation tank itself, and in order to provide a sufficient quantity of gas bubbles to produce a flotation effect of significant magnitude, the effluent must be recycled in an amount equivalent to 100 percent or more of the input into the flotation tank. This recycling of the flotation tank effluent necessarily requires additional tank and pumping capacity.
In one form of the dispersed air flotation technique, air may be introduced into the suction side of a pump that directs a part of the activated sludge from the settling tank to a special flotation tank. When this liquid containing added air undergoes a typical cavitation effect in the line from the pump, small air bubbles, ordinarily about 1 mm. in diameter, will be introduced into the special flotation tank and form a float with some of the suspended solids, which float then rises to the top of the tank. In other types of dispersed air equipment (exemplified by the three patents referred to above), air is introduced into an aqueous suspension of solid particles and dispersed throughout the liquid medium by the beating action of various shaped impellers, resulting again in bubbles that are about 1 mm. in diameter.
Whether it is the dissolved air flotation or the dispersed air flotation technique that is used with a known flotation tank of the type that follows the settling tank, substantially all the air bubbles introduced into the contents of the tank operate to raise suspended solids to the surface of the tank, and only a negligible proportion of the oxygen in the bubbles is available for additional biological oxidation of the activated sludge that is introduced into the flotation tank from the settling tank. The particular physical limitations of these two techniques that produce this result will be discussed below.